Don't validata filesystem target type

Don't validata filesystem target type

When using QEMU's 9pfs the target "dir" element is not necessarily an
absolute path but merely an arbitrary identifier. So validation in that
case currently fails with the misleading

   $ virt-xml-validate /tmp/test.xml
   Relax-NG validity error : Extra element devices in interleave
   /tmp/test.xml:24: element devices: Relax-NG validity error : Element domain failed to validate content
   /tmp/test.xml fails to validate